\documentclass[12pt]{article} 
 \textheight15cm \textwidth11cm \parindent0mm \parskip 1ex plus2ex minus0.8ex
\begin{document}
The layout of the following list is explicitly set up
(here you can see the textwidth of the current text):
\begin{list}{!}{\leftmargin 20ex \rightmargin 15ex \itemsep 0.5ex}
 \item Here is an item with a long text, so that you can see the width 
       of an item.
 \item Here is another item.
\end{list}

\newcounter{mycount}
The following list is also explicitly designed:
\begin{list}{\arabic{mycount}}{\itemsep 0.5ex\usecounter{mycount}}
 \item Here is an item with a long text, so that you can see the width 
       of an item.
 \item Here is another item.
\end{list}

The following list is also explicitly designed:
\begin{list}{!}{\leftmargin 20ex \rightmargin 15ex \itemsep 0.5ex}
 \item[abc] Here is an item.
 \item Here is another item.
\end{list}

The following list is also explicitly designed:
\begin{list}{\arabic{mycount}}{\itemsep 0.5ex\usecounter{mycount}}
 \item[def] Here is an item.
 \item  Here is another item.
\end{list}

The following list is a \verb|trivlist| environment:
\begin{trivlist}
 \item[a] Here is an item.
 \item[b] Here is another item.
 \item[c] And one more item.
\end{trivlist}
\end{document}
